Author: Jürgen Schnieders

Over the next few decades, the global building stock will have to change in such a way that it becomes compatible with a climate-friendly energy system. Many countries have signed declarations of intent to this effect. In Germany, there is the Federal Climate Protection Act for example, which requires net greenhouse gas neutrality by 2045. At the EU level, within the framework of ETS-2 it is foreseen that from the first half of the 2040s onwards, CO₂ certificates for marketing of combustibles will no longer be issued. An even faster pace would be necessary for complying with the Paris Climate Agreement of 2015, which is binding under international law and according to which global warming is to be limited to well below 2 degrees. Net greenhouse gas neutrality will certainly have to be achieved in the long term.

A country would be well advised to actively reduce its consumption of fossil fuels also for reasons other than climate protection. As demonstrated once again by the example of Russian natural gas in 2022, concerns about supply security, political susceptibility to blackmail and economically problematic cost fluctuations are by no means unfounded.

Regardless of whether climate neutrality in the building sector can realistically be achieved by 2045, 2050 or 2070, all efforts must be geared towards minimising cumulative CO₂ emissions by that time so that any remaining emissions ideally remain below the available emissions budget. Theoretically, there are many different ways to achieve the goal of greenhouse gas neutrality. One indispensable component, of course, is the generation of renewable energy; similarly important fields of action in the building sector are energy efficiency and the conversion of supply systems. The expansion of renewable energy sources in Germany should already be accelerated in 2023 through various legislative changes. The following scenarios assume an increase in renewable electricity generation to over 1000 TWh/a in 2050 (total electricity generation in 2023: 514 TWh).

Significantly improving the energy efficiency of existing buildings with the help of Passive House components is a building block with a number of advantages. The EnerPHit standard defined by the Passive House Institute is the benchmark for an appropriate quality of renovation.

Figure 1 illustrates the scope for action in the building sector based on the above assumption for renewable electricity generation. According to the 2020 legislation, the available budget (green bar) would be significantly exceeded. Further federal policy ambitions in 2021 aimed at improving thermal protection in renovations and new builds and increasing the share of renewable energy sources in the heat supply system. Assuming that implementation went according to plan – which it did not – this would have brought it just within the available budget. However, even better results would be achieved with consistent utilisation of the potential on the building side, i.e. a combination of the Passive House standard in new builds and Passive House components in existing building renovations (“EnerPHit”). The building sector could then remain within this budget even if e.g. only 50% expansion of renewables can be achieved.

Figure 1: Cumulated CO₂ emissions for heating and hot water from 2021 to 2070. The green band indicates the range for the greenhouse gas budget for the building sector that is still available. Illustration based on [PHI 2022].

Through the consistent use of Passive House components, the heating demand of existing buildings can be reduce by a factor of around 4. For this, it is important that the efficient components are used in an event-related way: if building components need to be renewed or replaced anyway, this is the right time to achieve a drastic improvement in efficiency at minimal additional cost, then these measures will also be the best choice economically.

As can be seen in Figure 2, it will take several decades to significantly reduce the heating demand of existing buildings in this way, but accelerating the process would hardly be financially viable as existing values would be destroyed. With such an accelerated path, the demand for materials and manpower would also rise sharply at first, but would fall again just as quickly after ten to twenty years. In addition, increased speed is likely to come at the expense of quality; the result would be a building stock of average quality which it would not be possible to finance any further - a dead end. The right approach is therefore to link very good thermal protection measures to the usual renovation cycles.

Figure 2: Existing building stock in Germany assuming different efficiency scenarios up to 2070. Illustration based on [PHI 2022].

Passive House technologies have proven their effectiveness time and time again for decades. They require hardly any maintenance or adjustment, and the risk of operating errors has proven to be negligible in practice. Their effectiveness does not diminish even after many years, as demonstrated by measurements e.g. in the Passive House building in Darmstadt-Kranichstein after 25 years of operation for example [PHI 2016].

Figure 3: Natural gas consumption in the Passive House building in Darmstadt-Kranichstein over 25 years [PHI 2023].

The lower consumption of EnerPHit buildings is achieved through improvements to components that are already needed anyway. The additional costs compared to a standard building renovation are therefore comparatively low. As already mentioned above, detailed analyses show that improved efficiency in building renovation is cost-effective if it is coupled with the usual renovation cycles, so that the moderate additional costs of e.g. a higher insulation thickness or better windows are essentially incurred for saving energy, but a high quality will then be worthwhile. As can be seen from [PHI 2022], the total costs, investment plus energy over the life cycle are lowest if the EnerPHit standard is consistently applied according to the coupling principle.

Figure 4: Energy and investment costs in various scenarios examined based on the example of Germany. Illustration taken from [PHI 2022].

In economic terms, there is also the fact that, as the savings achieved lead to lower CO₂ emissions, in the EU they also help avoid any penalties imposed on the country for failing to meet climate targets. At the same time, they reduce the cost of CO₂ emissions in sectors such as aviation and shipping, where they are more difficult to avoid.

In the European climate, space heating in winter accounts for a major share of the energy consumption of buildings. However, renewable energy is mainly available in summer and seasonal storage is therefore necessary, which leads to considerable losses. The PER demand shows how much renewable energy must be generated to cover the demand of a specific application.

Figure 5: PER demand in various scenarios examined based on the example of Germany. Illustration taken from [PHI 2022].

This is shown in Figure 5 for the year 2070, together with the potential for renewable energy generation. By combining Passive House/EnerPHit with heat supply that is adapted to a renewable energy system, the PER demand becomes very small. This leaves more renewable energy for applications that are more difficult to decarbonise.

In a renewable energy system, only two options that actually come into consideration as energy sources for space heating: electric heat pumps and district heating. The latter is suitable in urban neighbourhoods that still have a correspondingly high heat demand density even after energy retrofits of the buildings. Direct combustion of renewable hydrogen, methane from power-to-gas or similar for space heating would be far too inefficient and expensive. Based on the current annual trend in gas consumption, [Feist 2024] now shows that if a conservatively estimated 70% of the current building stock were to be supplied using heat pumps, the capacity of the electricity grid would have to be almost doubled. In addition, this peak output may coincide with a period of low energy generation, meaning that peak-load power plants would have to be kept available for this purpose, for example gas turbines that only run for a few hours a year. Both would significantly increase the costs of a renewable supply system. The higher the efficiency of the buildings, the fewer modifications to the grid will be necessary for ensuring the costly space heating.

Figure 6: Seasonal variation in gas consumption in Germany. Illustration taken from [Feist 2024].

A similar conclusion is reached if we consider the total electricity consumption of the previously mentioned heat pumps over the main heating period of 4 months instead of the peak output. During this period, there is hardly any sunshine in Central Europe for generating PV electricity. This contradictory behaviour of the space heating demand and renewable energy generation makes saved kilowatt hours in the heating sector particularly valuable.

The electricity consumed must be provided - at least on average over the heating period - mainly through wind energy. Imported renewable energy, for example in the form of ammonia, hydrogen or methane, would be significantly more expensive. Again, [Feist 2024] clearly shows that around 100 GW of installed wind energy capacity would be required to supply heat pumps in the unrenovated building stock, 145% more than was installed by the end of 2023. By consistently applying the efficiency path described above, i.e. high efficiency in conjunction with the coupling principle, this demand could be halved over a period of 25 years, which would eliminate many difficulties in the expansion and distribution of wind energy.

The distant future is naturally difficult to predict. How will the expansion of renewable energy infrastructure progress? What will happen to energy consumption in other sectors such as industry and transport? How will changes in international politics affect the energy supply of buildings? An existing building stock, the useful energy consumption of which has been reduced by half, which is especially comfortable, healthy and resistant to crises, with a flexible and cost-effective energy supply that fits into a climate-neutral energy system, will always create scope for development for a country; an EnerPHit renovation according to the coupling principle is a no-regret measure.

[Feist 2024] Feist, Wolfgang: Increase in electrical load in the grid through a systematic heat pump strategy in Germany; https://passipedia.org/basics/energy_and_ecology/increase_in_electrical_load_in_the_grid_through_a_systematic_heat_pump_strategy_in_germany

[PHI 2022] Jürgen Schnieders, Wolfgang Feist, Benjamin Krick, Jan Steiger, Witta Ebel: Towards a climate-compatible building stock https://outphit.eu/media/filer_public/c4/62/c46248f3-4b2e-45e6-adf3-3520b9d7579f/outphit_klimaneutralergebaudebestand_en_final.pdf|

[PHI 2016] Passive House – a permanent solution. A quarter of a century of experience
https://passipedia.de/beispiele/wohngebaeude/mehrfamilienhaeuser/ passivhaus_die_langlebige_loesung|

[PHI 2023] https://passipedia.org/examples/residential_buildings/multi-family_buildings/central_europe/the_world_s_first_passive_house_darmstadt-kranichstein_germany|

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